This invention relates to a light weight, thermally activated, electrochemical power supply. More particularly this invention concerns itself with a thermally activated, pelletized, reserve battery comprising a single cell, or a stacks of individual cells, which utilizes copper (II) chloride and an alkali tetrachloroaluminate as electrolytic ingredients.
The present interest and development of light weight communication systems for airborne applications has generated a considerable research effort in an attempt to provide light weight power sources for use in those systems. One type of power source which has proven effective is that which is thermally activated. The thermally activated characteristics of this power source provide the communication system with a power source of essentially unlimited storage life, rapid and reliable activation, and the ability to withstand the stress and strain encountered within an operational aerospace environment.
Considerable interest in the development of thermally activated galvanic cells has taken place in the aerospace field since World War II. Early cells utilized a "cup and cover", or similar design, which was followed by the development of a "pelletized cell". Further research in pellet cell technology took place with the eventual fabrication of a completely pelletized thermal battery for use in a variety of electrochemical systems. These power sources utilized electrolyts of inorganic salts that remained solid and inactive at storage temperatures. The power source or electrochemical cell was activated by heating the cell to an elevated temperature. The inorganic salt electrolyte melted under elevated temperature conditions and became ionically conductive. The elevated temperatures necessary for activation was generally provided by pyrotechnic heat sources such as heat powders or heat papers.
Generally, current production thermal batteries are characterized by the utilization of a LiCl-KCl eutetic electrolyte, an operational temperature within the range of about 400.degree. C. to 600.degree. C., and a lifetime ranging from a few seconds to several minutes. Recent workk has shown that current thermal battery systems can operate for as long as an hour under suitable conditions. This improvement in activated lifetime indicates that thermal batteries are desirable power source for long life applications, including guided bombs, missiles, ECM devices, and torpedoes. However, a disadvantage of present thermal battery systems for long life applications is the 400.degree. to 600.degree. C. operating temperature range. The high operating temperature range necessitates heavier insulation to attain the extended lifetime and to prevent overheating of adjacent electronic components.
During recent years, problems have also arisen with certain chemical components required for thermal batteries. For example, health hazards associated with Cr(VI) compounds and shortages of some raw materials has led to renewed interest in alternate electrochemical couples for the present electrolyte system. Completely new electrolyte systems and couples for thermal battery applications are also desired. Amongst the desirable characteristics for these new systems are a lower operating temperature which would reduce insulation requirements, battery volume, and conserve heat producing materials required for activation. Also, the avoidance of Cr(VI) compounds for the cathode would eliminate a primary carcinogenic health hazard.
Several electrolytes have been investigated to evalute their applicability to low temperature thermal batteries. For example, it has been determined that a 70:30 m/o (mole %) KSCN-NaSCN electrolyte in conjunction with various lithium metal alloy anodes and a V.sub.2 O.sub.5 cathode shows promise for high spin artillery shell applications. A heat reservoir to stabilize operational temperatures in a KSCN-NaSCN electrolyte system also has been developed. Unfortunately, these materials as well as other known materials, do not overcome the problems recited above.
With the present invention, however, it has been discovered that Copper (II) chloride provides an excellent candidate for a long life, pelletized, thermal battery system. Based on the evaluation of other cathodes, it has been determined that the open circuit and load voltages can be enhanced by utilizing Copper (II) chloride as a cathode in a LiAl/NaAlCl.sub.4 /CaCl.sub.2 cell coupled with a maximum energy density in the 200.degree. to 255.degree. C. temperature region.